You can put this solution on YOUR website! Imagine the triangle is made of sticks with hinges
at the corners.
Lay the 20 cm side flat on a table. The other 2 sides
must make an arch over the 20 cm side. But that is
impossible, because if you add 5 cm and 10 cm,
that is less than 20 cm, and together they are unable to meet
over the 20 cm side.